Thermodynamics Flashcards
What is the difference between enthalpy and enthalpy change?
Enthalpy is absolute whereas enthalpy change is the difference between the initial and final enthalpy
With what is enthalpy change measured?
Thermometer
What is enthalpy change?
Heat energy change measured at constant pressure
What is standard enthalpy change?
Heat energy change measured at constant pressure under standard conditions
What are the standard conditions?
298K
100kPa
all compounds and elements in their standard states.
What is the enthalpy of atomisation?
The enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state
What is the enthalpy of bond dissociation?
The enthalpy change when 1 mol of a covalent bond is broken to form 2 moles of gaseous atoms.
What is the formula for enthalpy of atomisation, relating enthalpy of bond dissociation?
Enthalpy of atomisation = enthalpy of dissociation/2
What is the enthalpy change of first ionisation?
The enthalpy change when 1 mole of gaseous +1 ions are formed from 1 mole of gaseous atoms
Describe the energy change in enthalpy of first ionisation
Endothermic
Energy is used to overcome electrostatic attraction between the positive nucleus and negative electron
What is enthalpy change of second ionisation?
The enthalpy change when 1 mole of gaseous +2 ions are formed from 1 mole of gaseous +1 ions
What is electron affinity?
Likeness for electron affinity
What is the enthalpy change of first electron affinity?
The enthalpy change when 1 mole of gaseous -1 ions are formed from 1 mole of gaseous atoms
Describe the energy change in the first electron affinity
Exothermic
Energy is released from the electrostatic attraction between the nucleus and electron being added
What is the enthalpy change of second electron affinity?
The enthalpy change when 1 mole of gaseous -2 ions are formed from 1 mole of gaseous -1 ions.
Describe the energy change in the second electron affinity
Endothermic
Energy required to overcome the repulsion between the negatively charged ion and electron being added
What is the enthalpy change of formation?
The enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions
What is the enthalpy of lattice formation?
The enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions
Describe the energy change in enthalpy of lattice formation
Exothermic
Energy is released from electrostatic attraction between oppositely charged ions
What is the enthalpy of lattice dissociation?
The enthalpy change when 1 mole of solid ionic compound is broken into its gaseous ions
Describe the energy changes in lattice dissociation
Endothermic
Energy is needed to overcome the electrostatic attraction between oppositely charged ions
How to distinguish between values of lattice enthalpy of formation and dissociation
Lattice formation has -∆H
Lattice dissociation has +∆H
What does lattice enthalpy indicate?
The strength of an ionic bond
Why can lattice enthalpies not be directly measured?
It is difficult to measure temp rise in a solid
Why is the Born-Haber cycle used to calculate enthalpy change?
Impossible to carry out experiments in which gaseous ions are spread out at infinite distances
Reactions take place in the gaseous state, gas particles move randomly, so difficult to bring particles close together to measure.
What factors affect lattice enthalpies?
Ionic charge
Ionic radius
How does ionic radius affect lattice enthalpy?
Smaller ions pack closely in a lattice, so attract strongly. Large ions are further apart in their lattice, so forces of attraction are weaker.
How does ionic charge affect lattice enthalpy?
The larger the ionic charge the stronger the attraction, so the greater the lattice enthalpy
What is theoretical lattice enthalpy?
Lattice enthalpy values calculated using the perfect ionic model (theoretical refers to the lattice enthalpy that is expected)
What is experimental lattice enthalpy?
The lattice enthalpy values from a Born-Haber cycle
What is the perfect ionic model?
Assumes ions are point charges and there is only electrostatic attraction (ionic bonding) between ions with no covalent character
What does it mean if theoretical and experimental lattice enthalpies are similar?
The compound has almost 100% ionic character (almost no covalent)
What is polarisation?
When the cation attracts the electrons of the anion, thus distorting the density in the anions towards the cation (hence the anion is polarised)
What is polarising power?
The ability of a cation to attract electrons from the anion towards itself
What is polarisability?
The ease with which an anion can be polarised
What are the properties of a cation with large polarising power?
Small with big charge
What are the properties of an anion with large polarisability?
Large with big charge
Why does covalent character lead to different theoretical and experimental values?
Covalent character leads to stronger bonds (?)
